1. Field of the Invention
The present invention relates to a headrest lock structure with lock slots, a method for forming lock slots in a headrest pole and a method of producing a headrest pole.
2. Description of the Related Art
FIGS. 8A and 8B schematically show the structure of a conventional automobile seat, wherein FIG. 8A is a perspective view generally showing the external appearance of the seat, and FIG. 8B is a perspective view showing a headrest of the seat.
As shown in FIG. 8A, the conventional automobile seat comprises a seat cushion 501 to be mounted on an automobile body, a seat back 502 attached to the seat cushion 501 in such a manner that it can be selectively reclined forwardly or backwardly relative to the seat cushion 501, and a headrest 503 attached to the top end of the seat back 502 in a vertically movable manner. As shown in FIG. 8B, the headrest 503 includes a cushion member 504 and a reverse U-shaped headrest pole 505 supporting the cushion member 504. The right and left legs 506, 507 of the headrest pole 505 are detachably inserted into a pair of corresponding pole guides 508 provided in the seat back 502.
The left leg 506 of the headrest 503 has an outer peripheral surface formed with a plurality of lock slots 509 which are arranged in a line along the axial direction of the left leg 506, and the pole guide 508 corresponding to the left leg 506 is provided with a lock plate 516 (see FIG. 9) adapted to be releasably fitted into either one of the lock slots 509. According to the above lock structure, the lock plate 516 of the pole guide 506 can be fitted into one of the lock slots 509 formed in the left leg 506 of the headrest pole 505 to hold the headrest 503 at a desired height.
FIGS. 9A to 9D show a conventional headrest lock structure, wherein FIG. 9A is an explanatory cross-sectional view of a production method for the headrest pole, FIG. 9B being a vertical-sectional view of the headrest pole which is engaged with the lock plate in its correct position, FIG. 9C being a cross-sectional view of the headrest pole and the lock plate in FIG. 9B, and FIG. 9D being a cross-sectional view of the headrest pole and the lock plate in the state after the headrest pole is rotated relative to the lock plate from the position in FIG. 9C.
Heretofore, a solid round bar has been used as a material of the headrest pole 505 and subjected to a cutting process to form lock slots 509 therein. Late years, in view of a relatively high production cost in the above technique, a pipe having a circular cross-section has been increasingly used as a material of the headrest pole 505 and subjected to a press-forming process using a punch 511 to form the lock slots 509 therein, as shown in FIG. 9A (see, for example, Japanese Patent Laid-Open Publication No. 15-009992).
The punch 511 for use in the above method has a front end 511a with a flat shape, and thereby each of the lock slots 509 is formed to have a flat bottom surface 512 and an upper side surface 513 extending outwardly from the bottom surface 512 at an approximately right angle therewith, as shown in FIG. 9B. In this figure, the reference numeral 514 indicates an arc-shaped portion (sagging portion) 514 connected to the upper side surface 513, and the reference numeral 515 indicated an inclined portion formed to have a given oblique angle relative to the bottom surface 512. Each of the lock slots 509 other than the lowermost lock slot has a vertically asymmetrical shape with the inclined portion 515, and only the lowermost lock slot 509 has a vertically symmetrical shape without the inclined portion 515 (as indicated by the two-dot chain line in FIG. 9B). The lock plate 516 is formed in a flat plate shape in which a lock portion to be brought into contact with the bottom surface 512 of each of the lock slots 509 has rectangular corners. The upper side surface 513 has an arch shape in top plan view (as indicated by the hatching in FIGS. 9A, 9C and 9D) defined between the arc-shaped portion 514 and the flat bottom surface 512, and the width of the upper side surface 513 has large changes ranging from a maximum width d1 to a minimum width 0 (zero).
Thus, if the headrest pole 505 is attached to the pole guides 508 in its given correct position, the lock plate 516 will be brought into contact with a portion of the upper side surface 513 having the maximum width d1 in one of the lock slots 509, so that an engagement region S1 between the upper side surface 513 and the lock plate 516 can have a desired area determined by the maximum width d1 and a maximum length L1 of the upper side surface 513, as shown in FIG. 9C.
While the method as disclosed in the above Patent Publication 1 can produce the headrest pole 505 through a simplified operation as compared with the conventional technique to achieve an extremely low-cost structure, a bending process of the headrest pole 505 to be performed after the formation of the lock slots 509 is liable to cause an angular error in the headrest pole 505. Further, the lock plate 516 includes production errors.
Due to the angular error of the headrest pole 505 caused by the bending process and the production errors of the lock plate 516, the headrest pole 505 is likely to be attached in a displaced or mismatch position where the headrest pole 505 is rotated from its given correct position. Moreover, an impact force is likely to be applied to the headrest pole 505 in a direction causing the rotation thereof (as indicated by the arrow “a” in FIG. 9D). In these cases, as shown in FIG. 9D, the lock plate 516 is brought into contact with the outward end portion 517 of the upper side surface 513 of one of the lock slots 509, or a portion of the upper surface 513 having a width less than the maximum width d1, so that an engagement region S2 between the upper surface 513 and the lock plate 516 has a width d2 which is reduced to about one-half of the maximum width d1 and a length L2 which is reduced as compared with the maximum length L1. Consequently, the area of the engagement region S2 is significantly reduced, and the headrest lock becomes liable to be unintentionally released. While the angular error caused by the bending process can be suppressed by performing the bending process before the press-forming process of the lock slots 509, the press-forming process performed after the bending process inevitably involves a time-consuming operation due to the obstructive curved portion of a pipe 510, resulting in difficulties in achieving a simplified production process.